Solid, you may recall, is the project by Tim Berners-Lee and company to create a distributed social web. This page makes it clear just how difficult and complex that task is. It's a 'roadmap' to becoming a Solid developer - that is, it identifies and links to the different things you need to know. The 'basics' include identity, DOM manipulation, APIs and more, not to mention graph data and RDF. You'll also need package managers like Yarn or (Node.js) NPM. Then you can move on to React, a Javascript framework for dynamic web interfaces. After that, you can begin to learn Solid itself.